Elemental addition to a material to reduce its non-dissolved carbide content while increasing its resistance to temper softening has helped to increase internal hardness after nitriding while maintaining its cold-coiling-ability. Conventionally, high strength has been achieved by applying nitriding as a surface treatment to the valve springs, without consideration of the compound layer. However, when the strength of wire coils, the hardness of the nitrogen-diffused layer, and internal hardness are maximally increased, the white layer becomes the points of origin of breakages, and fatigue strength does not increase. The white layer was therefore judged detrimental to fatigue strength, and a nitriding method was developed that attained nitriding without the white layer and the achievement of increased hardness in the nitrogen-diffused layer, which had previously been incompatible.
Article of Honda R&D Technical Review Vol.22 No.1 (68 - 75)